If you're using a touch enabled display and your default browser is Chrome the "On Drag Stop" event doesn't work when you're using your mouse to trigger the drag drop. The event will work if you use the touch screen to drag drop. This mixes the interactions between mouse and touch which makes it very confusing for the prototype testers.

Workaround is to set your browser in JIM to Firefox or IE (sorry I just threw up a little at recommending IE) BUT this doesn't help if your prototype testers are using the chrome on a touch enabled display, they'll have to use Fire Fox to use your On Drag Stop events...